You mean a mod that creates a relationship between two sims that didn't have one previously? They'd probably need to know each other in some way first, but I'm certain that's possible in some fashion (and indeed, mods to change relationship levels already exist). Almost any type of mod is possible, though - it just requires the right level of skill and knowledge. If you're interested in making mods, mosey yourself on over to the Create section for info and help on that. If you're just looking for preexisting mods, the Help section has a Where Can I Find sub-section that you can ask for mods to do what you're looking for.